Process of making photo-electric tubes



Dec. 5, 1933. w, KLUGE 1,938,431

PROCESS OF MAKING PHOTO ELECTRIC TUBES Filed Kay 18, 1931 2 E 4 20 D14 E E /0-- e00 730 7150 630 600' J50 J06 450 400 11 Inventor: Werner KI ug His Afit'orney Patented Dec. 5, 1933 U ITED STATES PATENT OFFICE Werner Kluge, Bcrlin-Hermsdorf,

Germany,

assignor to General Electric Company, a. corporation of New York Application May 18, 1931-, Serial No. 538,081, and in Germany May 21, 1930 1 Claim. (Cl. 25027.5)

It is known to use alkali metals, and preferably potassium, for tubes operable on the photoelectric efiect. When the tubeis used for industrial purposes, the photo-electric electron 5 emission of the layer of pure potassium is too small so that the said layer must be sensitized according to the process described by Elster and Geitel. Hydrogen is introduced into the tube at low pressure and a glow discharge is thereafter 9 caused to take place for a short period between the electrodes of the tube. A layer of potassium hydride is formed for the most part and is recogmzable by its blue-violet color. In the best of tubes, the photo-electric output over the light spectrum, i. e., the number of electrons released froma layer of this kind (and in relation to the wave length) for each incident monochromatic unit of light energy is shown by the curve 1 in the accompa drawing. There is no photoelectric action above the wave length 550 known as the red border.

lilydrides have proved to be very unstable as a photo-electric sensitive substance. They disintegrate under the action of light and are dissociated by the positive ions of the rare gases of the tube, the said gases impinging on the light-semitive layer when the load is put on the tube. This layer evaporates when heated (about '10"). For the e reasons, the life of tubes of this kind is 39 comparatively shortand the defect is particularly troublesome when the tubes are used in sound film machines, in which capacity they should stand a long-period load withunvarying sensitiveness.

The subject-matter oi the invention is an improved process ior the manufacture of photoelectric tubes which are free from the defects mentioned in the foregoing. The principle oi the invention is explained hereunder.

40 A portion of the inner wall or the vessel of the tube is first silvered according to a known process, for example by placing in the said vessel a solution oi silver nitrate and adding a reducing solution thereto which is measured so that the silver is preciptated at the desired point on the glass wall. The tube is new connected to thepump and is exhausted while simultaneously heated to about 370 C. tor a long period of time in order to expel gas and dampness residues liable to affect the operation of the tube detrimentally at a later stage. Dry oxygen is now admitted to a pressure of fractions of one millimetre of mercury and a pulsating glow discharge produced through the tube by applying thereto a direct-current voltage of about 400 with the aid of a suitable series resistance. The discharge is maintained until a visible alteration in the layer of silver indicates the presence of an adsorbed layer of oxygen. After being thus treated, the layer of silver assumes a yellowish tint. The foundation layer for the alkali metal being now ready, the said metal is now distilled, through the pump connection, into the envelope preliminarily treated in this manner. The quantity of alkali metal is such that the foundation layer becomes faintly dull. If voltage is now applied to the tube, a photoelectric current may be produced by illumination. According to the invention, the sensitiveness of the tube, especially in the infra-red region of 7 the spectrum, may be further increased in the following manner. The tube is heated to about 200 C. for a few seconds, preferably by means oi a large flame surrounding the whole tube and the atomic potassium vapor is adsorbed on the layer or oxygen. Particularly good results are obtained by additionally ionizing thepotassium vapor within the tube. To this end, a voltage or about 100 volts is applied during the short period of time within which the tube is heated. Elec- 89 trons are liberated by illumination of the cathode to produce potassium ions by collision on their way to the anode. This ionizing current may be measured by means of an ammeter disposed in the load circuit, but of course remains in existence only for a short time because the potassium vapor gradually flows back to the distilling apparatus through the pump connection. The positive alkali metal ions are adsorbed on the oxygen layer and form a highly sensitive 90 layer. The distilling apparatus is cooled as soon as this ionizing current is detected by the ammeter and'the non-ionized and non-adsorbed vapor is caused to return to the distilling apparatus by further heating the tube. The cooling action takes eflect immediately behind the pump connection by introducing compressed air, whereal'ter the production of the light-sensitive layer is completed; the tube may now be illled with rare gas and sealed off.

The spectroscopic output of the improved photo-electric tube provided with a layer of this kind is shown by curve No. 2 in the drawing. This curve is of considerably greater amplitude in all parts of the spectrum than in the case 0! a layer of potassium hydride. The positive ion current of the rare gas which impinges on the light-sensitive layer when the tubes are put under load, produces no alteration in the emitting power. Also, the properties of the layer are not the tubes is thus much longer and answers industrial requirements.

The sensitiveness in the red zone of the spectrum is remarkable because photo-electric action may be detected at about 800ml. A tube improved in accordance with the present invention may'also be used as a receving instrument for modulated infra-red waves in ultra-short wave telephony. s

While I have described my'invention more particularly with reference to potassium as the light-sensitive material, it is to he understood that the improved process is also applicable to alkali metals in general, including sodium, caesium and rubidium.

What I claim as new and desire to secure by Letters Patent 0! the United States, is:

In the manufacture of a photoelectric tube, the method which comprises the following steps, coating the interior of the envelope with a foundation layer of metal, introducing oxygen into the tube, passing a glow discharge to oxidize the metal layer, admitting alkali metal into the tube, heating the tube to vaporize the alkali metal and applying a voltage to the tube during the heating period to cause current to flow through the tube and to sensitize the alkali metal.

WERNER KLUGE. 

